1. Field of the Invention
The present invention relates to pulse width modulating device and an exposure device, and in particular, to a pulse width modulating device which can generate pulse width modulating signals having a high bit resolution and an exposure device to which the pulse width modulating device is applied.
2. Description of the Related Art
FIG. 16 illustrates a general structural example of a conventional pulse width modulating device which generates pulse width modulating signals (hereinafter, “PWM signals”).
In a pulse width modulating device 80 illustrated in FIG. 16, in accordance with a reset signal generated by a clock generator 82, a set signal is inputted to a set terminal (S terminal) of an R-S flip-flop 86 by a control logic 84, and the output signal from an output terminal (Q terminal) of the R-S flip-flop 86 rises. The reset signal generated by the clock generator 82 is inputted to a counter 88 so that the counter 88 is reset.
Thereafter, counting by the counter 88 of an operation clock generated by an operation clock generator 90 begins. A comparator 92 compares this counted value with PWM data which expresses the pulse width of each pulse of a PWM signal which is to be generated. When the counted value and the PWM data are equal, with a signal outputted from the comparator 92 as a trigger, the control logic 84 outputs a pulse to a reset terminal (R terminal) of the R-S flip-flop 86. The output signal of the R-S flip-flop 86 thereby falls, and the PWM signal which is to be generated is obtained.
However, in the above-described conventional pulse width modulating device, as described above, the pulses of the operation clock are counted, and the timing of the fall of the PWM signal is determined on the basis of the results of comparison of the counted value and the PWM data. Therefore, the bit resolution of the generated PWM signal (the number of bits of PWM data which can be reproduced by the maximum pulse width of the PWM signal) is determined by the frequency of the operation clock, and a drawback arises in that the determined bit resolution cannot be improved.
Namely, in a case in which, for example, the frequency of the operation clock is 40 MHz and the maximum pulse width of the PWM signal is 102.375 μS, at a unit of 25 nS (= 1/40 MHz), a resolution of only 4095 (=102.375 μS÷25 nS, 12 bits) can be obtained for the pulse width.
Accordingly, when such a pulse width modulating device is applied to an exposure device which carries out exposure in accordance with the pulse width of a PWM signal, a problem arises in that the accuracy of the exposure time cannot be improved above the accuracy determined in accordance with the frequency of the operation clock.